Covered welding electrode

ABSTRACT

A COVERED WELDING ELECTRODE HAVING AN AUSTENTIC ALLOY CORE WIRE CONTAINING LESS THAN 0.01% CARBON AND ESSENTIAL FOR ACCEPTABLE CREEP-RUPTURE DUCTIBILITY OF WELD METAL IN THE AS-DEPOSITED CONDITION.

United States Patent 3,554,791 COVERED WELDING ELECTRODE Edwin W.Johnson, Murrysville, and Frederick C. Hull, Pittsburgh, Pa., assignorsto Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation ofPennsylvania No Drawing. Filed Oct. 4, 1968, Ser. No. 765,037 Int. Cl.B23k 35/22 US. Cl. 117-205 11 Claims ABSTRACT OF THE DISCLOSURE Acovered welding electrode having an austenitic alloy core wirecontaining less than 0.01% carbon and essential for acceptablecreep-rupture ductility of weld metal in the as-deposited condition.

CROSS REFERENCE TO RELATED APPLICATIONS This application is related tocopending application Ser. No. 765,159, filed on Oct. 4, 1968.

BACKGROUND OF THE INVENTION Field of the invention This invention isdirected to covered welding electrodes prepared from a core wire of afamily of austenitic stainless steel alloys and welds made therefrom.The weld deposits of this alloy are characterized by an exceptionalcombination of high creep-rupture strength and good rupture ductility.The welds are further distinguished by the fact that although they havea fully austenitic microstructure, they are highly resistant to hotcracking or microfissuring during the welding of restrained joints.Another highly desirable feature of these alloys is that the freedomfrom hot cracking of the welds is obtained without any preheating, whilethe high creep-rupture strength and stress-rupture ductility of thewelds are obtained without the use of any post-weld heat treatment.

DESCRIPTION OF THE PRIOR ART The properties of creep-rupture strengthand ductility are important when alloy parts are subjected to elevatedtemperatures of operation. To increase efficiency in steampower plants,for example, there is a long-term trend toward the use of higher steamtemperatures. At the lower steam temperatures used in the past, thepiping through which the steam is conducted was fabricated from lowalloyferritic steels which were satisfactory. At the higher steamtemperatures used in some of the more modern plants and contemplated formore extensive use in the future, the mechanical properties of ferriticsteels are less favorable, due primarily to the fact that the strengthof such steels declines rapidly within increasing temperature. Withincreasing steam temperature, therefore, the ferritic steel pipingmust-have thicker walls for a given steam pressure. This results inincreased weight per unit length of pipe and requires the use of largerexpansion loops, heavier supporting structures and more welding, whichin turn results in greater costs.

It is well known that austenitic steels are much stronger than ferriticsteels at all temperatures above about 1000 F. However, the use ofaustenitic steel steam piping in the past has been characterized bymajor difliculties peculiar to austenitic steel itself. One problem hasinvolved weld hot cracking, wherein a weld bead tends to crack in abrittle manner at an early stage of cooling from its solidificationtemperature in the presence of ordinary stresses. This hot cracking hasgenerally been a more troublesome problem in austenitic steels than inferritic steels. Moreover, failures of welded joints in austenitic steelsteam piping after a period of routine service have Patented Jan. 12,1971 been found to occur under conditions symptomatic of severe orabnormal weakness and/or brittleness of the weld joint in astress-rupture mode of failure.

A nominal range of compositions of fully austenitic steels which aredisclosed in US. Pat. No. 3,201,233 is set forth in Table I as follows:

TABLE I Weight percent Chromium 1420 Nickel 15-30 Manganese 7.5-15Molybdenum 0.5-3.75 Mn and Mo total 9-16 Carbon 0.010.08 Nitrogen0.01-0.35 Silicon Up to 1 Vanadium Up to 0.3 Boron Up to 0.03 ZirconiumUp to 0.06 Iron Balance Steam piping made from the alloy composition setforth in Table I includes a carbon content of from 0.01 to 0.08%, and asilicon content of up to 1%.

In the prepartion of heats for production, however, it has been foundthat the air melted heats from which pipe is fabricated usually have anaverage carbon content of 0.03% and a silicon content of 0.15%. Thefully austenitric iron-base alloys having the compositions of Table Ihave been used as steam-turbine piping materials due to their highstrength and ductility at elevated temperatures and low susceptibilityto weld cracking. The usual method of welding such alloys has been thetungsten inert-gas (TIG) process. Sound welds with favorable mechanicalproperties have been produced in such alloys by the tungsten-inert-gas(TIG) rocess.

Although the TIG process can produce reliable welds either manually orautomatically, there are many instances when covered electrode weldingis preferred. For example, in field erection of equipment is confinedspaces, the greater accessibility of the manual covered electrode is adistinct advantage. Moreover, the covered electrode provides its ownprotective shield so that the often cumbersome inert gas lines and watercooling lines of the TIG process are not needed. Another advantage ofthe covered electrode process is that there are available more welderswho are qualified to use this process. Finally, the equipment for theTIG process, with all its automatic controls, is much more costly thanthe power supply for the covered electrode process.

US. Pat. 3,201,233 presents the stress-rupture properties of coveredelectrode Welds made with core rods of compositions within the range ofTable I, having coating fluxes of commercially available compositions.The rupture strengths were considerably lower than those of TIG weldsand the rupture ductilities of the covered electrode welds dropped withincreasing rupture time to unacceptably low values around 1% elongation.

Early attempts to weld the fully austenitic alloys by the manualcovered-electrode method resulted in unacceptably low values of thecreep-rupture strength and particularly low stress-rupture ductility ofthe as-deposited weld metal such as indicated in Pat. No. 3,201,233.They are now known to be primarily attributable to high concentrationsof carbon and silicon in the welds. The carbon and silicon contents ofthe covered-electrode welds were significantly higher than therespective concentrations of these same elements in TIG welds preparedfrom filler wires derived from the same alloy heats as the core Wires ofthe covered electrodes. The creep-rupture properties of the respectivecovered-electrode welds were far lower than those of the correspondingTIG welds.

Prior known commercially available coatings are the primary sources ofboth the carbon and the silicon contamination of the weld metal. Thecarbon is derived from carbonates, high-carbon master alloys and othersuch carboniferous constituents of the coating, while the silicon isderived from high-silicon master alloys such as ferrosilicon, from'various solid silicates such as clays, and from silicate binders.

A source of silicon contamination of the weld metal derived from theinvention is the silicate binder of the 4 DESCRIPTION OF THE PREFERREDEMBODIMENT To provide a covered-electrode weld between abutting ends ofsteam piping which weld has a high creep-rupture strength andsatisfactory rupture ductility, it is desirable that the core wire ofthe covered electrode have a carbon content of less than 0.01%. The corewire of this invention has composition ranges such as shown in Table IIas follows:

TABLE II.CORE WIRE COMPOSITION WEIGHT PERCENT Broad Option TypicalOption Typical range A A B B Chrimium 13 20 15-18 16 16 Nickel 19 M 2020 Manganese 7-15 10 10 Molybdenum. 0. 54 1. 753 2. 2 1. 75-3 2. 2Carbon Less than Less than 0 008 Less than 0. 006

0. 01 0. O1 0. 01 Nitrogen Up to 0.30-.. 0. 03-0. 14.... 0.08 0.15-0.250.10 Silicon Up to 0.4 Up to 0.25 0.10 Up to 0.25 0.10 Vanadium Up to 1-O. 10 to 0.35 0. 2 Boron Up to0.03 Up to0.02... 0 010 0. 005-0. 02...0.015 Zirconium- Up to 0.04. Up to 0.02 Iron 1 Balance.

electrode coating. This binder, which is constituted of sodiurn orpotassium silicate or a mixture of these silicates, is responsible for aweld-metal silicon content of from about 0.1% to about 0.5%, dependingon the other ingredients of the coating. The maximum stress-ruptureductility of the Weld metal is ultimately limited by this siliconcontamination originating from the electrode coating binder.

In the absence of any significant sources of carbon in the electrodecoating, the carbon content of the weld metal is ultimately determinedby the carbon concentration in core wire of the electrode. In thisrespect it has been found to be extremely desirable to use core wireshaving carbon concentrations below 0.01%.

It has been found in accordance with this invention that an unexpectedlysignificant improvement in the creeprupture strength and ductilityproperties of welds occurs from the use of covered welding electrodeswherein the carbon content of the core Wire of the electrode issubstantially less than 0.01%. This advantage is particularly importantwhere the base metal, the weld, and the core wire consist of fullyaustenitic steel. Moreover, the advantage of improved creep-rupturestrength and ductility is obtained in the as-deposited condition of theweld metal, and therefore the need for post-weld heat treatment iseliminated.

Accordingly it is an object of this invention to provide a coveredWelding electrode having an austenitic alloy core wire containing lessthan 0.01% carbon.

It is another object of this invention to provide a covered weldingelectrode which provides acceptable creeprupture strength and ductilityproperties of a weldment in the as-deposited condition.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

SUMMARY OF THE INVENTION Briefly, the invention comprises a coveredwelding electrode including a core wire containing less than 0.01% ofcarbon and a coating encasing the core wire, the coating being composedof an agglomeration of nonmetallic constituents including fluorides,oxides, and some metallic constituents, and the core wire being composedof an alloy comprising, by weight, from about 13 to 20% chromium, fromabout 13 to nickel, from about 5 to 18% manganese, from about 0.5 to 4%molybdenum, less than 0.01% carbon, up to about 0.30% nitrogen, up to0.4% silicon, up to 1.0% vanadium, up to 0.03% boron, up to 0.04%zirconium, and the balance being iron and incidental impurities. Alloywelding rod with a carbon content of 0.008% and less gives exce l nresults.

Generally, the fully austenitic weldable alloys for core wires of thisinvention comprise the compositions under the Broad Range lflOrl'n whichsatisfactory welds are obtained. For welds of strength at elevatedtemperatures comparable to that of AISI Type 316 stainless steel, alongwith excellent rupture ductility, the compositions listed under Option Aand Typical A are useful. The alloys of Option B and Typical B provideconsiderably higher weld strength with good rupture ductility atelevated temperatures.

The coating for the electrode which is applied directly onto the corewire in a suitable manner such as extrusion, is an agglomeration ofseveral ingredients held together by a suitable binder. The ingredientsof nominal compositions of coatings are listed in Table III inconcentration units of parts-by weight (p.-b.w.), where one (p.b.w.) isdefined as one weight percent of the total amount of only thenonmetallic ingredients of the coating, exclusive of water.

The chemical and physical functions of the various coating ingredientsare known only in varying degrees. CMC serves primarily as a slip agentfor aiding extrusion while the soluble silicate serves as a binder tokeep the coating intact until such time as the electrode is actuallyused in welding. The primary role of the manganese metal powder ispresumed to be that of a chemical reducing agent or deoxidizer, alloyingbeing an additional but secondary function. The fluorides, includingboth fluorspar and cryolite, are primarily fluxing agents that aid theimpurity-scavenging action of the molten slag. The fluorides as well asthe TiO and titanates also influence certain physical properties of theslag including the slags fluidity while molten and ease of removal aftersolidification. The small proportion of tferroboron (or otherboroncontaining material) has the primary function of introducing aparticular amount of boron into the weld deposit.

=Cr O and CaMoO are known to be sources of alloyed Cr and M0. Theresults of using other techniques of enriching the weld metal with thesame elements has led to the conclusion that the alloying eflects aloneare insufficient to explain the various metallurgical benefits gainedfrom the presence of Cr O and CaMoO in the coating. These compounds aresources of oxygen, the liberation of which have the effect of limitingthe contamination of the weld metal by oxidizable impurities includingcarbon, silicon, and sulfur. The beneficial effect of the CaMoO may bebased on the known volatility of M00 at the welding temperature. Thevolatilization of the M00 has two desirable effects. First, theliberated vapor would serve as a gas atmosphere shield to protect themolten weld metal from chemical attack by the air, acting in ananalogous way to the CO gas that is liberated from the CaCO used in manyconventional electrode coatings. Another benefit is that the residualcondensed material is CaO, which is a strongly basic ingredient of theslag that is extremely desirable for aiding the prevention ofcontamination of the weld metal by acid-oxide-fonming elements includingsulfur, phosphorous, and silicon. Most of the molybdenum initiallyintroduced in the form of CaMoO, was recovered in the deposit suggestingthat complete volatilization of the M does not occur. A combination ofall of these possible mechanisms is the best explanation for thebeneficial effects of the CaMoO on the mechanical properties of theas-deposited weld metal.

The core wire was coated by applying a flux coating to the wire byextrusion. The batch size of each flux coating was about 1 kg. whichyielded 60 to 70 electrodes per experiment. Three coatings Nos. 1, 4, 5,and 6, having compositions listed in Table III, were applied to the tenlots of core wire as shown in Table IV. The electrodes were baked attemperatures of up to 700 F. in two steps, the first of which occurredimmediately after extrusion and terminated at 475 F. The latter at ahigher temperature was performed just prior to welding.

Base materials were high strength stainless steels in the form ofV-grooved /8 inch thick plate. Each grooved plate was formed bylongitudinally sawing a 12" X 12" by /8 plate into two equal pieces 60from the main surface, inverting one piece end-over-end, and welding itbeside the other piece to a 1 inch thick backing plate.

TABLE IIL-COATING COMPOSITIONS PARTS BY WEIGHT Rutile (Ti02)....

1 Alkali silicate binder exclusive of H content. 2 A water soluble gumsuch as sodium earboxymethyl cellulose. 3 Balance.

The following example illustrates the practice of the invention.

EXAMPLE Ten lots of core wire for experimental electrodes were derivedfrom various heats in heat sizes varying from to 3000 pounds. Most Otfthe heats were prepared by partial vacuum induction melting, wherein themajor elements iron, nickel, chromium, molybdenum, and vanadium werevacuum melted together after which the furnace was filled with argon ornitrogen and then manganese and master alloys supplying the nitrogen,boron, and zirconium were charged. The heats were poured under argon ornitrogen at atmospheric pressure. The ingots were converted intostraight cut lengths of wire in commercial wire production facilities.The wire segments were usually finished to the final diameter bycenterless abrasion. The core wire diameters in all experiments were Thechemical wire analyses are listed in Table IV.

The grooved plates were filled by a down-hand manual welding procedurewith a reverse polarity DC are of 145 amp. with the inch electrodes. Tofill each groove requires about 16 passes of the 9, inch electrodes.

The as-deposited weld metal was converted into mechanical testspecimens, most of which were longitudinally oriented tensile specimensof gage diameter 0.357 inch and gage length 1.5 inch. The specimens Wereused in all of the tensile tests and most of the creep-rupture tests,particularly those expected to terminate in less than 1000 hours.

The creep-rupture properties of the as-deposited welds derived from theseveral electrodes are shown in Table IV.

The salient feature of Table IV is that the welds derived from corewires having carbon analyses of less than 0.1% displayed much highervalues of rupture elongation than did those from the core wires with acarbon concentration exceeding 0.01% when the same coating flux TABLEIV.CREEP RUPTURE PROPERTIES OF WELDS 1,200 F. TEST TEMPERATURE WeldDeposit Number Coating analysis 1 N0 1 No.1 No.4 No.4 No. 4 No. 5 No. 5No.5 No.5 No.6 No. 6

' 4 ht er t wuqlysis. can 15.1 15.1 15. 57 15. 3 15. 23 15. 39 15. 9514. 9 15. 47 14. 7 15. 3 Ni: 21. 9 20.1 22.8 21.4 21. 15 13.13 21.1221.0 20.51 21.5 21.2 Mn- 9. 95 9. s5 10. 15 11. 5 10. 02 10. 13 10. 9s13. 44 10. 52 10. s 10. 0 Mo.- 2. 25 1. 95 2. 23 2. 1s 2. 25 2. 34 2. 252. 14 2. 19 2. 05 2. 25 N. 0. 31 0. 13 0. 122 0. 15 0. 25 0. 15 0. 13 0.234 0. 21 0. 05 0. 25 v 0.19 0. 2 0. 2 0.155 0.15 0.18 0.07 0.19 0.15 B0. 003 0. 002 0. 010 0. 010 0. 009 0. 010 0. 011 0. 000 0. 013 0.007 0.009 Z17. 0. 003 0. 012 0. 003 0. 002 0.007 0. 103 0. 002 0. 015 0. 0150. 005 0.007 c 0. 053 .008 0. 041 0. 003 0. 005 0. 03 0. 004 0. 0250.005 0. 01s 0. 005 sf... 0. 00 .02 0. 03 0. 09 0. 05 0. 05 0. 02 0. 050. 02 0. 03 0. 05 s 0. 007 005 0. 007 0. 004 0. 003 0. 007 0. 010 0. 0050. 00s 0. 007 0. 003 P1111: .002 0. 01 0 001 0. 004 0.005 0. 004 0.0020. 004 0. 001 0.004 0 0.012 Al i i n 3 7 5 50'0' 50'0' 20"47'5""40'0""50'0 "37*"17'5 40.0 40.0 .5

iii'iii nire" 12 433 252 179 391 411 133 440 133 43s Elongation(percent).. 3. 3 9. 3 2. 7 18.7 19. 6 4. 7 28. 7 5. 7 28.0 9. 3 18.0Reduction of area (percent). 6. 0 24. 2 10.3 24. 0 19. 6 14. 4 43.0 9. 438.6 16.0 33. 4

1 Table III.

2 Iron base with incidental impurities.

was used with both wires. More specifically, deposit numbers 142B,286-A, 297-A, 325B, 328-A, and 296A were derived from core wires havinga carbon analyses of from 0.003 to 0.008%.

In view of the fact that the covered-electrode weld creep-ruptureproperties are sensitive functions of the electrode coating constitutionas well as of the core-wire composition, the arrangement of the columnsin Table IV is such that the properties of different welds derivedelongation value for No. 102 is 3.3% as compared with that of No. 142Bwhich is 9.3%.

Similarly the coating No. 4 was employed with a set of core wires having0.041% carbon and with two sets of core wires having 0.003% and 0.006%carbon, respectively. The rupture ductility properties of weld No.244-A, 286A, and 297-A, derived from core wire having carbon contents of0.041%, 0.003%, and 0.005%, respectively, are characterized byelongation values of 2.7%, 18.7%, and 19.6%. This confirms the sensitiveinverse response of the weld rupture ductility to the carbon content ofthe core wire. It particularly emphasizes the benefits obtained from theemployment of core wire containing less than 0.01% carbon.

For coating No. 5 involving weld deposit Nos. 325C, 325B, 301-A, and328A were derived from core wire containing 0.03%, 0.004%, 0.025%, and0.006% of carbon, respectively. The resulting elongation values are4.7%, 28.7%, 8.7% and 28.0%. These results are in accord with thesensitive inverse correlation indicated above with respect to coatingNos. 1 and 4.

In a similar manner, a coating No. 6 involved deposit Nos. 266B and296A, derived from core wires having carbon contents of 0.018% and0.006%, respectively. The resulting elongation values are 9.3% and18.0%, respectively, indicating that there is a definite inversecorrelation between carbon content and rupture ductility properties.

Accordingly, in accordance with this invention, superior stress-ruptureductility of the as-deposited weld metal is derived from covered weldingelectrodes in which the core wire carbon content is less than 0.01% andin which the coating on the core wire has a composition within theranges indicated hereinabove.

Various modifications may be made within the spirit of the invention.

What is claimed is:

1. A covered welding electrode for weld deposits characterized by highstress-rupture strength and good rupture ductility up to at least 1200F. and low cracking during welding, comprising a core wire and a coatingtherefor; the core wire comprising, by weight, from about 13 to about20% chromium from about 13% to about 30% nickel, from about 5% to about18% manganese, from about 0.5% to about 4% molybdenum, less than 0.01%C, up to about 0.30% nitrogen, up to about 0.5% silicon, up to about 1%vanadium, up to about 0.03% boron, up to about 0.04% zirconium, and thebalance being iron with incidental impurities, the coating comprisingfrom about 5 p.b.w. to 40 p.b.w. of a fluoride, up to about 30 p.b.w. ofa titanate, up to about 15 p.b.w. of CaMoO up to about 25 p.b.w. of Cr Ofrom about p.b.w. to p.b.w. of a silicate binder, up to about 1 p.b.w.of CaSiO up to about 1 p.b.w. of a water soluble gum, up to about 0.5p.b.w. of boron, up to about 10 p.b.w. of manganese, and the balancebeing rutile.

2. The electrode of claim 1 wherein the core wire contains from about0.0005% to 0.008% carbon.

3. The electrode of claim 1 wherein the core wire contains about 0.002%carbon.

8 from dilferent core wires, and tested with substantially identicalcoatings, can be conveniently compared. Coating No. 1 was employed ontwo sets of core Wire having clifferent carbon contents. Weld deposit,No. 102, involving one set which included 0.053% carbon and weld depositNo. 142-13 was derived from the other set of core wire containing 0.008%carbon. As indicated in the table the rupture ductility properties ofthe weld No. 102 are dis tinctly inferior to those of weld No. 142-3.Thus, the

4. The electrode of claim 1 wherein the core wire contains from about15% to about 18% chromium, from about 19% to about 24% nickel, fromabout 7% to about 15 manganese, from about 1.75% to about 3% molybdenum,up to 0.008% carbon, from about 0.03% to about 0.14% nitrogen, up toabout 0.02% boron, and the balance being iron.

5. The electrode of claim 1 wherein the core wire contains from about15% to about 18% chromium, from about 19% to about 24% nickel, fromabout 7% to about 15 manganese, from about 1.75 to about 3% molybdenum,up to 0.008% carbon, from about 0.15% to about 0.25% nitrogen, fromabout 0.10% to about 0.35% vanadiurn, from about 0.005% to about 0.02%boron, and up to about 0.02% zirconium.

6. The electrode of claim 1 wherein the core wire contains about 16%chromium, about 20% nickel, about 10% manganese, about 2.2% molybdenum,about 0.008% carbon, about 0.08% nitrogen, about 0.10% silicon, about0.01% boron.

7. The electrode of claim 1 wherein the core wire contains about 16%chromium, about 20% nickel, about 10% manganese, about 2.2% molybdenum,about 0.006% carbon, about 0.19% nitrogen, about 0.10% silicon, about0.2% vanadium, about 0.015% boron, and the balance being iron.

8. The electrode of claim. 1 wherein the coating comprises from about 10p.b.w. to about 35 p.b.w. of CaF up to about 5 p.b.w. CaMoO from about10 p.b.w. to about 15 p.b.w. of silicate binder, up to about 0.5 p.b.w.CaSiO and from about 0.1 p.b.w. to about 0.2% boron.

9. The electrode of claim 1 wherein the coating comprises about 30p.b.w. CaF about 10 p.b.w. titanate, about 5 p.b.w. CaMoO about 10p.b.w. Cr O about 11.5 p.b.w. of K2Si30q, about 0.5 p.b.w. of awater-soluble gum, from about 0.05 p.b.w. to about 0.2 p.b.w. of boron,and about 3 p.b.w. manganese.

10. The electrode of claim 8 wherein the core wire contains from about15% to about 18% chromium, from about 19% to about 24% nickel, fromabout 7% to about 15 manganese, from about 1.75 to about 3% molybdenum,from about 0.03% to about 0.14% nitrogen, up to 0.008% carbon, up toabout 0.02% boron, and the bal ance being iron.

11. The electrode of claim 8 wherein the core wire contains about 16%chromium, about 20% nickel, about 10% manganese, about 2.2% molybdenum,about 0.006% carbon, about 0.19% nitrogen, about 0.2% vanadium, about0.015% boron, and the balance being iron.

References Cited FOREIGN PATENTS 165,685 4/1950 Austria 117202 WILLIAML. JARVIS, Primary Examiner U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3: 55179 Dated January 97 Inventor(s) Edwin w. Johnson and Frederick C. HullIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2, line 40, cancel "is" and substitute in Column 6, line 32,cancel "0.1%" and substitute 0.01% Column 7, line 9, after, "derived"insert the first nine lines of column 8. Column 7, line 51, after"chromium" insert a comma. Column 8, lines 1-9, cancel.

Signed and sealed this 13th day of July 1971 (SEAL) Attest:

WILLIAM E. SCHUYLER,

EDWARD M.FI.ETCIER,JR.

Commissioner of Paton Attesting Cfficer

